On 9 Oct 2020 at 06:39:27 BST, "Roland Perry" wrote:

In message l.net, at
22:06:37 on Thu, 8 Oct 2020, Dave Liquorice
remarked:
On Thu, 8 Oct 2020 20:57:01 +0100, Roland Perry wrote:

Wouldn't put the stat in a room with a heat source it's not
controlling. Wood burner makes living room nice and warm, stat is
satisfied and rest of house doesn't get any heat.

But I'm not in any of the rest of the house, Dear Liza.

*Any* heat, even if it's blowing a gale and -10 C outside...

Remember with your current short cycling, if a room cools bit that
radiators thermostatic valve opens a bit and warms the room. With the
room stat satisfied this won't happen as the boiler is forced off.

It's a small enough house that heat percolates around it even when room
rads are off. But that wasn't really my question.

I understand about the short-cycling (but in practice the bathroom rad
and the one in the former sitting room, now my office, are both warm
almost all the time the heating is on), however if the room stat simply
switches off the boiler like a timer would, does the circulating pump
stop, and does in effect turning the boiler on and off several times an
hour have any undesirable other side effects.

I'm not sure I could find a separate connection to make to the boiler to
switch it by a thermostat rather than the timer. The timer just has one
2-core connection to the boiler, plus its mains power.

It's a combi boiler and hot water is provided on demand even if there's
no controller attached to the wall.

Then just put your thermostat in series with the controller. In principle if
the controller is at mains voltage it would be best to put the thermostat in
series with the live wire, but if they are both unearthed, double insulated it
doesn't really matter. And if they are both extra low voltage it also doesn't
matter which wire.

I suspect all modern boilers have pump overrun (and a permanent mains
connection even when the controller is off) to stop the main heat exchanger
getting too hot when switched off, and with old cast iron ones it doesn't
matter.

I am sure your boiler has a way of switching off on demand water heating,
which would save a few quid over a year by stopping short cycling to keep the
water heat exchanger hot if you don't mind not having hot water immediately
available at some hours, and save bits of the planet; but I have mine
permanently on too!


--
Roger Hayter